fix(ipc): back off failed process initialization

[swayamg20]

Summary

• add bounded exponential backoff from 1 to 30 seconds after failed idle-process initialization
• allow waiting jobs to bypass the cooldown only for the process capacity they require
• reset retry escalation after a successful initialization and cover failure, exception, cooldown, and resumption paths

Fixes #4857

Testing

• uv run pytest tests/test_ipc.py --unit -q — 16 passed
• make check
• uv run pytest --unit -q --ignore=tests/test_room.py — 1708 passed, 2 skipped (tests/test_room.py requires a local livekit-server binary)

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.